Periodic voltage generator



May 24, 1938. H. M. LEWIS 2,118,352

PERIODIC VOLTAGE GENERATOR Filed Feb. 12, 1936 FIG J.

IO l2 13 I4 I5 F ESL IE N EY QDETECTOR OII-'R\I{IE)UEEOI\J CYC RR AMPLIFIER AMPLIFIER TUBE o---o oo o I I I6 [To 0 PICTURE- FREQUENCY GENERATORC 536 FlG.4.

I a l JJIG so '1 24 0.0. I 1 4 32 5| gm 1 3 I e 'PI'I M a INVENT OR.

ATTORNEY.

Patented May 24, 1938 UNITED STATES PATENT OFFICE 2,118,352 PERIODIC VOLTAGE GENERATOR Harold M. Lewis, Douglaston, Long Island, N. Y., .assignor to Hazeltine Corporation, a corporation of Delaware Applied f-i February 12, 1936, Serial No. 63,574

5 Claims.

In television systems, according to present practice, a picture is transmitted on a carrier frequency modulated by picture or video-frequency signals corresponding in amplitude to the varying values of light and shade in the transmitted picture. A cathode-ray tube is employed at the receiver, wherein a cathode ray or. electron beam is produced and is focused upon a fluorescent screen. Voltages at the video frequencies, derived from the received carrier, are utilized to vary the intensity of the ray according to the signal amplitude. In order to effect scanning by' the ray, saw-tooth current or voltage waves are generated and utilized to produce, respectively, magnetic or electrostatic fields of saw-tooth wave form which defiect the cathode ray, usually horizontally for line scanning and vertically for picture scanning. The well-known rectilinear scanning pattern is thus traced upon the screen to reproduce the scene. Synchronizing impulses are also transmitted, usually also as modulation frequencies, impressed on the same television carrier as the video frequencies, and these impulses are utilized at the receiver to control or operate the scanning wave generators, thereby to synchronize the scanning action at the receiver with the corresponding actions at the transmitter.

As the art of television has. advanced, increased voltages have been employed forthe electrodes of the cathode-ray tube, as for focusing the ray, requiring increased scanning voltages. It is, of course, essential that the generated scanning voltages or currents be of the proper wave form to effect accurate linearityof trace and rapid retrace of the scanning ray, and at the same time it is highly desirable that the voltage generators employed for this purpose be relatively simple in construction and operation.

Because of inherent electrostatic and magnetic disturbances to which the deflecting plates of a. cathode-ray tube are subjected in operation, it is desirable, if not necessary, that these plates be electrically balanced with respect to ground, that is, have equal impedances to ground. With such an arrangement the disturbances produce eflects of equal magnitude and similar phase on both deflecting plates and, their effect on the scanning action of the tube is eliminated. With the plates so balanced with respect to ground, the deflecting voltages applied thereto should also be balanced relative to ground, that is, applied to the plates with equal amplitudes and opposite polarities, if maximum"efliciency and fidelity of reproduction is to be obtained. More particularly, in the case of an electron tube scanning wave generator, by utilizing two deflecting voltages balanced with respect to ground, the voltage of the direct-current supply need be only one-half that which would be required if a single unbalanced scanning voltage were developed and utilized. Furthermore, any electrostatic and magnetic disturbance's to'which the volt-supply circuits are subjected will be balanced out or neutralized in these circuits also, when such a balanced arrangement is employed a It is an object of the present invention, therefore, to'provide an improved generator for developing a periodic voltage of balanced saw-tooth wave form, preferably of relatively high ampli tude, and especially suitable for use in a tele-' vision receiver to efiect scanning by a cathoderay tube.

It is a further object of the invention to provide a generator of the character described which is relatively simple in its construction and operation.

In accordance with the present invention, there is provided a generator including means for generating a first periodic voltage of saw-tooth wave form having a predetermined polarity relative to ground. The generator includes also repeating means proportioned and arranged to derive from the first periodic voltage a second periodic voltage having substantially the same amplitude and wave form as the first voltage but of opposite polarity relative to ground. Means are coupled" to the voltage generating means and to the repeating means for utilizing the two balanced periodic voltages; In accordance with one embodiment of the invention, the first periodic voltage generating means comprises capacitance means provided with charging and discharging circuits. A constant current device is preferably included in one of the circuits for maintaining the current therein substantially constant. Means are employed for controlling the circuits to eflect a. periodic charge and discharge oi the capacitance means" thereby to produce the first periodic voltage." I

The repeating means, in the preferred e'mbodi-- ment of the inventlon,. .comprises 'a' reversing vacuum tube amplifier having an input circuit connected across a portion'oi the capacitance means. 1

A regenerating tube may also be employed for accelerating the action of the control tube to shorten the retrace time or to make the generator self-sustaining. A voltage divider arrangement.

may be utilized for simultaneously controlling the effectiveness of the synchronizing impulses and the regenerating action in opposite senses to obtain the desired operation.

For a better understanding of the present invention, together with other and further objects.

vision receiving system including a scanning voltage generator embodying the present invention; Fig. 2 is a graph of the balanced saw-tooth voltages developed by the generator of the invention as shown in Fig. 1; Fig. 3 is a circuit dia-' gram of a modified form of the generator of Fig. 1; and Fig. 4 is a graph of the balanced sawtooth voitages developed by the generator shown in Fig. 3.

Referring now particularly to Fig. 1 of the drawing, the system there illustrated comprises a television receiver, which may be of either the tuned radio-frequency or superheterodync type, including, in cascade, an antenna and ground system Ill-I l, a carrier-frequency amplifier I2, a detector i3, a video-frequency amplifier M and a cathode-ray tube I5. Where the receiver is of the superheterodyne type, the conventional oscillator-modulator and intermediate-frequency amplifier will be included as a part of the carrierfrequency amplifier i2. A picture-frequency generator |6and a line-frequency generator,indicated generally by the numeral H, are also coupled to the video-frequency amplifier and connected to the scanning elements or plates of the cathoderay tube in a conventional manner. The generator H, which is constructed in accordance with the present invention, is shown in detail and will be hereinafter further described. The tube I5 is preferably conventional in construction and operation.

',Since the system of Fig. 1, thus far described, is, in general, well understood in the art, a detailed explanation of its operation is deemed unnecessary. Briefly, however, the vision-modulated carrier waves are intercepted by the antenna ||i-|| and selectively amplified in the carrier-frequency amplifier .12. Where the receiver is of the superheterodyne type, the signals will be impressed upon an oscillator-modulator in ,the amplifier |2 where they will be converted to intermediate frequencies and further selectively amplified. The amplified signal from the ampli- .fier I2 is delivered to the detector l3 in which are derived the video frequencies of modulation, including the synchronizing components, which are supplied to the video-frequency amplifier M, wherein they are amplified and supplied, in the usual manner, to the control grid of the cathoderay tube IE and to the control elements of the "scanning generators l6 and |'l.- The intensity of the electron beam or ray of the cathode-ray tube I5 is thus modulated or controlled in accordance with the video-frequency voltages impressed on a control electrode of the tube, in the usual manner. The saw-tooth current and voltage scanning waves, which are generated in the picture-frequency generator l6 and linefrequency generator I I, respectively, are controlled by the synchronizing voltage'impulses supplied from the video-frequency amplifier l4, and are utilized in the conventional manner to produce magnetic and electro-static fields of saw-tooth wave form to deflect the ray vertically and horizontally, so as to trace the rectilinear scanning pattern upon the screen and thereby reproduce the transmitted picture.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention, the voltage generator I! comprises capacitance means, which in this form of the invention is made up of two condensers l8 and IS in series. Charging and discharging circuits are provided for the condensers. The charging circuit comprises a suitable source of direct current,

as, for example, a battery 20 which is preferably grounded at its negative terminal, a resistor 22 and the space-current path of a vacuum tube 2|. The tube 2| is effective to control the charging circuit to effect a periodic charge and discharge of the capacitance means. The discharging circuit, which is eifectively connected across the condensers .l8 and IS in series, includes an impedance element whereby the condensers will be discharged substantially linearly with respect to time. In the embodiment of the invention shown in Fig. 1, this element comprises a pentode tube 25 having suitable potentials applied to its electrodes from the battery 20 to effect operation thereof as a constant current device, in a well known manner.

There is also provided, in connection with the tube 2|, a regenerating tube 26 having its cathode grounded and its anode connected to the control electrode of the tube 2|. The control grid of the tube 26 is connected, by way of a suitable coupling condenser 21, to theadjustable tap of the voltage divider 23, one end of which isconnected to the anode of the tube 2| and the other end of which is coupled to ,the, video-frequency amplifier l4 through a condenser 24. A suitable biasing resistor 28 is connected between the control grid and cathode of the tube 26 and operating potential is supplied to the anode of this tube from a positive terminal of the battery 20 by way of a resistor 29.

For developing the second saw-tooth voltage, repeating means is provided which preferably comprises a reversing amplifier tube 30. The input circuit of the tube 30 is connected across a part of the capacitance means; more particularly, the cathode of the tube 30 is grounded by way of a suitable biasing resistor 3| and by-pass cone denser 32, and the control electrode or grid of the tube is connected to the junction between the condensers l8 and IS. A suitable biasing resistor 33 is connected between the control electrode and cathode of the tube 30. Operating voltage is supplied to the anode of the tube 30 from the battery 20 by way of a suitable resistor 30a. The junction between the condenser |8 and'anode of the tube 25, and the anode of the tube 30 may be connected directly to the scanning elements of the cathode-ray tube l5 throughv coupling condensers 36 and 31, respectively, or, as shown, to the opposite ends of a pair of serially-connected resistors 34 and 35 having their junction grounded for periodic currents, the opposite terminals of these resistors being connected to the scanning elements. The junction of resistors. and 35 may be grounded for direct current also, as shown, or may be maintained at any suitable direct voltage with reference to the operating voltages of the electrodes of the cathode-ray tube.

In the operation of the generator shown in Fig. l. the condensers Hi and I9 are rapidly charged In Fig. 2, curves 38 and 39 indicate the periodic from the source 20 by way of the tube 2| and are discharged substantially linearly and at a slower rate through the tube 26. The discharging action continues until the potential difference developed across the tube 2| is sufiicient to break down the tube and initiate a charging action through this tube. The initial surge of charging current results in a voltage impulse across the resistor 22 which is impressed negatively, by way of the voltage divider 23 and condenser 21, upon the control grid of the regenerating tube 26, re-. ducing the conductivity of this tube. A positive impulse peak is thereby developed across the resistor 29 and applied to the control grid of the tube 2| to accelerate the charging action. In other words, the tube 26 regenerates the tube 2| during the charging of the condensers i8 and l3.

Synchronizing voltage impulses are supplied from the video-frequency amplifier by way of the condensers 24 and 21 and voltage divider 23 to the control grid of the tube 26, preferably poled so that the peaks are negative at the grid of 26. The circuit constants are so selected that the voltage built up across tube 2| during the trace isijust short of breaking down this tube with normal grid voltage applied. During this portion of-the cycle the grid voltage of the tube 26 remains constant, so that its anode voltage, which is applied directly to the grid,of the tube 2|, is also constant. Upon the occurrence of a negative synchronizing impulse, the conductance of the tube 26 is suddenly decreased and its anode voltage raised. That is, the grid voltage of the tube 2| is increased positively to such a valuethat the existing anode voltage is sufficient to break it down. In brief, the synchronizing impulses serve to maintain the charging and discharging actions of the generator in proper synchronization with the corresponding actions at the transmitter.

Adjustment of the tap of the voltage divider 23 serves to vary simultaneously and in opposite senses the effectiveness upon the control grid of the tube 26 of the synchronizing voltage impulses from the video-frequency amplifier l4 and the voltages developed across the resistor 22 during the charging or retrace portion of'the cycle. It will be understood that suitable discriminating means will be included-in the input circuit of the generator l6 or I! or both to ensure that each is controlled only by its respective synchronizing impulses.

The voltage developed across the condensers l8 and I9 is applied by way of the condenser 36 and the resistor 34 to one of the scanning elements of the tube l5, as above described. The input circuit of the reversing amplifier tube 3|: being connected across the condenser |9,'the voltage across this condenser controls tube 30 and there is produced in the output of the amplifier 30 a voltage which is of the same wave form and'amplitude as the voltage developed across the condensers l6. and I9, but which is oppositely poled relative ground. This second periodic voltage is applied by way of the condenser 31 and the resistor 35 to the scanning element of the tube l opposite the 'first-mentioned'scanning element; Since the circuit constants are proportioned so that the peak periodic voltages developed are only a small fraction of the voltage of the source for instance, of the order of a few per cent, such non-linearity or inequality of amplitude as would result from the difierence in operating voltages of the two condenser circuits is minimized to such an extent that it is inappreciable.

tive to ground.-

In Fig. 3 there is illustrated a modified form of the generator shown in Fig. 1, wherein the apparatus is somewhat simplified. No regenerating means are employed in this embodiment of the invention although it will, of course, be understood that regenerating means similar to that included in the generator of Fig. 1 may be utilized if desired. The generator of Fig. 3 is arranged to produce voltages across the respective output resistors 34, 35 of polarities which are 0pposite to the polarities of the voltages produced across the corresponding resistors in the generator of Fig. 1. Elements of the generator of Fig. 3 which are similar to elements shown in Fig. 1 are given like reference numerals. Thus, inthis modified generator, the charging circuit of the condensers I8 and I9 includes the voltage source or battery 20, as in Fig. 1, and a constantcurrent device indicated schematically at 25a,

The discharging circuit comprises the spacecurrent path of the tube 2|a. The device 25a may be a' pentode tube with proper operating potentials applied to its electrodes, as in Fig. 1, or any other equivalent device. A suitable biasing potential is applied to the control grid of the tube 2|a by way of a resistor 40. The sync ronizing voltage impulses from the video-frequency amplifier M are applied directly to this control grid by way of the coupling condenser 24. A reversing amplifier tube 30 is here arranged in substantially the same manner as in the embodiment of Fig. 1, a suitable load resistor 4| being provided in the operating potential supply lead from the battery 20 to the anode of this tube." The output circuit of this generator includes coupling condensers 36, 31 and resistors 34, 35, as in the embodiment of Fig. 1.

The graph shown in Fig. 4 is similar to that .of Fig. 2, the curves 42 and 43 indicating the voltages developed across the resistors 34 and 35, respectively, of the generator of Fig. 3. The abscissae and ordinates are the same as in Fig. 2, and the polarities are similarly indicated.

While there have been described what at present are considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without. departing from the invention,'an;l it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A' periodic voltage generator comprising means for generating a first periodic voltage having a predetermined polarity relative to ground and including as a functional part thereof an impedance element having a point of intermediate potential, voltage repeating means including an input circuit, a direct connection between said input circuit and said circuit element at said 7 point of intermediate potential, the repeating characteristic of said repeating means being so related to its coupling to said generating means as to cause said repeating means to produce a second periodic voltage of substantially the same amplitude and wave form as said first voltage but of opposite polarity relative to ground, an output circuit for said generator, and means for connecting said output circuit to said generating means and said repeating means so that said two periodic voltages appear therein as balanced oppositely poled voltages.

2..A periodic voltage generator comprising capacitance means, charging and discharging circuits for said capacitance means, means for controlling said circuits to effect aperiodic charge and discharge of said capacitance means thereby to produce a first periodic voltage having a predetermined polarity relative to ground, repeating means having an input circuit connected across at least a part of said capacitance means and having a repeating characteristic so related to its input circuit connection as to produce a second periodic voltage of substantially the same amplitude and wave form as said first voltage but of opposite polarity relative to ground, an output circuit for said generator, and means for conmeeting said output circuit to said capacitance means and to said repeating means so .that said two periodic voltages appear therein as balanced oppositely poled voltages.

3. A periodic voltage generator comprising a plurality of condensers connected in series, charging and discharging circuits for said condensers,'

means for controlling said circuits to efiect a periodic charge and discharge of said condensers thereby to produce across said condensers a first periodic voltage having a predetermined polarity.

relative to ground, voltage amplifying means having its input circuit connected across at least one of said condensers and having a repeating characteristic so related to its input circuit connection as to produce in its output circuit a second periodic voltage of substantially the same amplitude and wave form as said first voltage but of opposite polarity relative to ground, an output circuit for said generator, and means connecting said output circuit to said condensers and said output circuit of said voltage amplifying means so that said two periodic voltages appeartherein as balanced oppositely poled voltages.

4. A periodic voltage generator comprising a plurality of condensers in series, charging and discharging circuits for said condensers, means for controlling said circuits to effect a periodic charge and discharge of said condensers thereby to produce across said condensers a first periodic voltage having a predetermined polarity relative to ground, a repeating tube having its grid-cathode circuit connected across one of said condensers and a resistor in its anode circuit for producing across said resistor a second periodic voltage of substantially the same amplitude and wave form as said first voltage but of opposite polarity relative to ground, an output circuit-for said generator, and means connecting said output circuit to said condenser and said resistor so that said two periodic voltages appear therein as balanced oppositely poled voltages.

5. In a television receiving system including a cathode-ray tube, a periodic scanning voltage generator for said tube comprising means for generating a first periodic voltage having a predetermined polarity relative to ground and including a circuit element having a point of intermediate potential, repeating means including an input circuit, a direct connection between said input circuit and said circuit element at said point of intermediate potential, the repeating characteristic of said repeating means being so related to its coupling to said generating means as to cause said repeating means to produce a second periodic voltage of substantially the same amplitude and wave form as said first voltage but of opposite polarity relative to ground, an

output circuit for said generator, and means connecting said output circuit to said generating means and said repeating means so that said two periodic voltages appear therein as balanced oppositely poled voltages.

HAROLD M. LEWIS. 

